Closed loop motor control circuits for controlling the positioning of a valve, damper, actuator, or similar load device are well known in the prior art. Moreover, some prior motor control circuits included devices for adjusting the electronic stroke of the motor used to drive the load device. By including stroke adjustment devices, these control circuits attempted to make it possible for a user to adjust the magnitude of the rotation of the load device. However, the design of many of these electronic stroke adjustment circuits was such that adjustment of the electronic stroke caused variations on both ends of the stroke. Therefore, if a particular load device required a fixed position, such as a fixed closed position of a damper in a heating and cooling system, these motor control circuits were not entirely suitable because adjustment of the electronic stroke would alter the closed position of the load device.
In an attempt to correct this problem, specialized electronic stroke adjustments for heating and cooling system motor controls were developed, such as that shown in U.S. Pat. No. 3,824,439 to Pinckaers. The '439 patent discloses a closed loop motor control circuit based on a balanced Wheatstone bridge and having a motor which controls the position of a load device such as a valve or damper from a fixed closed position to an open position. This circuit also provides an electronic stroke adjust which attempts to allow the adjustment of the open position of the valve for a given control signal without any change of the fixed closed position.
Referring to FIG. 3 which shows the circuit of the '439 patent, a closed loop motor control circuit is shown having a control potentiometer 138 and a feedback potentiometer 130 connected in a balanced Wheatstone bridge 120, and an electronic motor control 122 which senses the bridge output and drives the motor 124 depending on whether the bridge is balanced. An extremely important feature of the '439 control circuit is that in order for bridge 120 to be balanced, a wiper 128 of feedback potentiometer 130 must be at a virtual center point 134 of bridge 120 when valve 126 is at the fixed closed position, so that the resistive values of feedback potentiometer 130 and a resistor 140 are identical.
In the control circuit of the '439 patent, when wiper 128 of feedback potentiometer 130 is at virtual center point 134, bridge 120 is symmetrical and the potential at terminals 142 and 152 connecting a stroke adjust potentiometer 160 are the same. Therefore, stroke adjust potentiometer 160 has no effect on the bridge output signal, and any adjustment of potentiometer 160 will not affect the closed position of valve 126. When wiper 128 of feedback potentiometer 130 moves off virtual center point 134 to rebalance bridge 120, stroke adjust potentiometer 160 adjusts the value of the bridge impedance. Thus, in order to rebalance bridge 120 for two different settings of stroke adjust potentiometer 160, the motor 124, valve 126 and wiper 128 must drive farther with a lower setting of stroke adjust potentiometer 160.
The design of the motor control circuit disclosed in the '439 patent has proven problematic in several respects. For example, the control circuit does not include a device for calibrating the zero setting or fixed closed position of the valve. As is often possible, if valve 126 is not properly positioned at the desired closed position for a corresponding control signal, the zero setting must be calibrated. In order to do this without mechanical adjustment of the feedback potentiometer, some type of electrical zero setting device connected to bridge 120 would be useful for calibrating the zero setting of valve 126. However, because the control circuit uses a balanced Wheatstone bridge circuit, the zero setting device most likely will cause bridge 120 to become unbalanced when the zero setting of valve 126 is calibrated, or will alter the gain of the bridge with respect to given control signals. As stated above, when bridge 120 is not balanced, any adjustments to stroke adjustment potentiometer 160 will affect the bridge output signal and can therefore alter the fixed closed position of valve 126. If the zero setting and stroke adjustment settings affect each other, as they likely will in this type of arrangement, calibration of the system can become more complex, requiring a higher level of skill on the part of the technician, or a lower expectation of proper calibration on the part of the user.
In most heating and cooling systems, it is expected that the motors and their control circuits eventually must be replaced. However, the use of prior art motor control circuits for replacement purposes further emphasizes the problems discussed above. For example, when a replacement motor control circuit must be provided, variables such as the unknown wiring resistance of the heating and cooling system can significantly affect the operation of the control circuit. Because of such problems, it becomes even more difficult to provide a replacement control module which operates so that stroke adjustments can be made which do not alter the fixed closed position of a load device. Further, although control circuits such as that shown in the '439 patent often have been used as replacement controls, these control circuits do not allow the performance of a quick and uncomplicated calibration of the zero setting of a load device which is independent of stroke adjustments.